We have shown that the CD13/APN peptidase is a critical regulator of endothelial cell function and is required for angiogenesis. During endothelial cell activation, CD13/APN expression is induced by tumor-derived growth factors. These signals are transmitted to the nucleus where they enhance the binding of a potentially novel transcription factor to the CD13/APN promoter that is critical for its expression and therefore, its function. Treatment of cells with NSAIDs inhibits the induction of CD13/APN by interfering with the binding of this inducible transcription complex. The fact that NSAIDs directly affect CD13/APN expression suggests that CDI3/APN can serve as a surrogate for NSAIDs-modulated tumor neovascularization and may serve as a marker of treatment efficacy. Because CD 13/APN is also expressed on a subset of breast tumors where we have shown that its expression is regulated as it is in endothelial cells, it is likely that this CD13/APN expression in tumors reflects the dysregulation of normal, NSAIDs-sensitive signaling pathways. Therefore, we propose that NSAIDS treatment will modulate CD13/APN expression in breast tumor cells as well, suggesting that CD13/APN may also be a useful biomarker of NSAIDs prevention in these tumors. We hypothesize that the efficacy of NSAIDS chemoprevention of at-risk breast carcinoma patients can be monitored by assessment of CD13/APN expression either in serum or biopsy specimens. Furthermore, we find that CD13/APN cell surface expression significantly correlates with breast cancer cell invasion and therefore CD13/APN positive breast cancers may comprise a uniquely invasive and NSAIDS-sensitive subgroup. In these tumors we propose that inhibition of CD13/APN expression will inhibit tumor invasion. Finally, NSAIDS functional interference with a novel angiogenesis-induced transcription factor presents a new target for tumor directed therapy. The legitimacy and prognostic potential of predictive biomarkers is dictated by the accuracy and strength of the mechanistic link between a treatment, its specific effect at the site of action, and the desired therapeutic outcome. Therefore, this molecular relationship between chemoprevention, transcriptional modulation, and therapeutic effect warrants further investigation. The experimental plan outlined in this proposal will characterize in detail the features of CD13/APN as an NSAIDS modulatable surrogate marker for the chemoprevention of breast carcinoma, its contribution to tumor invasion and growth, and the molecular mechanisms controlling its regulation.